1. Field of the Invention
This invention relates to a process for preparing a catalyst which involves coating a support with a first solution containing a platinum or palladium salt and a rhodium salt, calcining, further coating the calcined support with a second solution containing a salt of ruthenium and an oxide, salt or acid of phosphorus and then further calcining.
2. Description of the Prior Art
During normal operation of an internal combustion engine, such as that of an automobile, the principal undesirable components in the exhaust gas, namely, nitrogen oxides, carbon monoxide and unburned hydrocarbons, that are discharged into the atmosphere can be reduced in amount by first passing the exhaust gas over ruthenium and/or rhodium under a reducing atmosphere to convert the nitrogen oxides selectively to nitrogen and then passing the treated gas, together with added oxygen, over platinum and/or palladium under an oxidizing atmosphere to convert the carbon monoxide and unburned hydrocarbons to carbon dioxide and water. By "reducing atmosphere" we mean an atmosphere wherein the stoichiometric ratio of molecular oxygen to reducing agents in the reaction zone is less than 1:1, preferably about 0.9:1 or even less. By "oxidizing atmosphere" we mean an atmosphere wherein the stoichiometric ratio of oxygen to reducing agents (for example, carbon monoxide plus hydrocarbons) is at least about 1:1, preferably about 2:1 or more.
During startup or cold start, however, when the engine is cold and operation thereof is effected under closed choke where the air to fuel ratio is lower than stoichiometrically required with a small amount of air, the engine exhaust contains insignificant amounts of nitrogen oxide but significantly larger amounts of carbon monoxide and unburned hydrocarbons than would be present during normal operation. Similarly, the ruthenium or rhodium catalysts and the platinum or palladium catalysts are at ambient temperature and will not begin to perform their intended functions until reaction temperature is reached. Since the amount of nitrogen oxides present during startup under closed coke is small, no problem exists during such period because of the inactivity of ruthenium. However, the presence of larger amounts of carbon monoxide and unburned hydrocarbons in the exhaust gas from the beginning of operation requires that they be converted immediately to the innocuous carbon dioxide and water. However, since the platinum catalyst is located at a further distance from the engine, it will require a relatively long time for the exhaust gases to raise the temperature thereof to a level at which a desired oxidation of carbon monoxide and unburned hydrocarbons takes place. Until such temperature level is reached the undesired carbon monoxide and unburned hydrocarbons will be discharged into the atmosphere.
Ruthenium, on the other hand, is closer to the engine manifold and it will reach a higher temperature level sooner than platinum. Therefore, it might be expected that during startup oxygen could be introduced into the exhaust gas prior to its contact with ruthenium so that the ruthenium could serve as oxidation catalyst for the desired conversion of carbon monoxide and unburned hydrocarbons. Thereafter when the platinum catalyst reached reaction temperature, the oxygen flow could then be switched to the mixture entering the platinum reaction zone. Unfortunately, ruthenium is not a satisfactory oxidation catalyst in the defined reaction, for its reactivity is too low to obtain sufficient oxidation at low temperatures.
In our U.S. Pat. No. 3,965,040 we claimed a process for preparing a catalyst which involves coating a support with a first solution containing a platinum or palladium salt, calcining, further coating the calcined support with a solution containing a ruthenium or rhodium salt and an oxide, salt or acid of phosphorus and then further calcining.